“Pyrophoric Iron Sulphides
Pyrophoric iron sulphides form when iron is exposed to hydrogen sulphide, or any other compound that contains sulphur, in an oxygen deficient atmosphere. They are found frequently in vessels, storage tanks, and sour gas pipelines. Pyrophoric iron sulphides present a hazard when equipment and tanks are opened for cleaning, inspection, and maintenance. As the iron-sulphide compounds dry out and come in contact with air, they react with the oxygen and spontaneously ignite.
The reactivity of an iron sulphide depends on the type of iron oxide from which it was derived. Reactive iron sulphides can be deactivated when wetted with oil, therefore, rusted surfaces that are below the oil level are at low risk of causing an explosion.
Chemical and mechanical methods are available to remove iron sulphides. The use of potassium permanganate is gaining acceptance for this purpose because it improves safety, saves significant cost and increases productivity. Other treatments include acid washing, chemical suppression, and the use of high-pH reagents.
General Precautions to Avoid Pyrophoric Iron Fires
1. The scraps and debris collected from cleaning of filters in naphtha / crude service must be kept wet and disposed of underground.
2. Tanks, reactors, columns, and exchangers in high-sulfur feed service must be kept properly blanketed with N2 during idle periods.
3. All equipment and structured packing must be properly water washed and kept wet when exposed to the atmosphere.
4. In processes where catalyst handling is required (such as in Hydrotreating and fluid catalytic cracking) caution must be taken during catalyst recharge or disposal. When unloading any spent coked catalyst, the possibility exists for iron sulfide fires. If the spent catalyst is warm and contacts oxygen, iron sulfide will ignite spontaneously and the ensuing reaction may generate enough heat to ignite carbon deposited on the catalyst. Therefore catalyst must be stripped of all hydrocarbons, cooled to about 50 o C and wetted with water to prevent it from igniting vapors. Once cooled, the used catalyst may be emptied into drums for later shipment to a regenerator or a disposal site. As the catalyst may be highly pyrophoric (containing iron sulfide, etc.), it should be dumped into drums containing an internal liner for shipment. The drum and liner should first be filled with inert gas, which is then displaced by the catalyst. The liner should be tied off and a small chunk of dry ice placed inside the drum before sealing. These precautions should protect against catalyst auto ignition.”
